In 3GPP LTE, OFDMA (Orthogonal Frequency Division Multiple Access) is employed as a downlink communication method. In a radio communication system adopting 3GPP LTE, a base station transmits a synchronizing signal (synchronization channel: SCH) and a broadcast signal (broadcast channel: BCH) using prescribed communication resources. A terminal first synchronizes with a base station by capturing the SCH. Then, the terminal acquires parameters that are specific to that base station (for example, the frequency bandwidth) by reading BCH information (see Non-patent Literature 1, 2 and 3).
Also, after the terminal acquires base station-specific parameters, the terminal sends a connection request to the base station, and, by this means, establishes communication with the base station. When necessary, the base station transmits control information to the terminal, with which communication has been established, using a PDCCH (Physical Downlink Control CHannel).
The terminal performs “blind detection” for the received PDCCH signal. That is, a PDCCH signal includes a CRC (Cyclic Redundancy Check) part, and, at a base station, this CRC part is masked by the terminal ID of the target terminal. Thus, until a terminal demasks the CRC part of a received PDCCH signal with the terminal's terminal ID, the terminal cannot decide whether or not the PDCCH signal is for that terminal. In this blind detection, if the result of demasking is that CRC calculation is OK, the PDCCH signal is decided to be sent for the terminal.
Also, control information sent by a base station includes assignment control information including, for example, information about resources which a base station allocates to a terminal. A terminal needs to receive both downlink assignment control information and uplink assignment control information which have a plurality of formats. Although downlink assignment control information which a terminal should receive can be defined in a plurality of sizes depending on the transmitting antenna control method and frequency allocation method at a base station, some of these downlink assignment control information formats (hereinafter simply referred to as “downlink assignment control information”) and uplink assignment control information formats (hereinafter simply referred to as “uplink assignment control information”) are transmitted using PDCCH signals of the same size. A PDCCH signal includes type information of assignment control information (for example, a 1 bit flag). Thus, even if the size of a PDCCH signal including downlink assignment control information and the size of a PDCCH signal including uplink assignment control information are the same, a terminal checks type information of assignment control information, and by this means can distinguish between downlink assignment control information and uplink assignment control information. The PDCCH format to transmit uplink assignment control information is PDCCH format 0, and the PDCCH format to transmit downlink assignment control information, transmitted in a PDCCH signal of the same size as for uplink assignment control information, is PDCCH format 1A.
However, cases might occur where the information size of uplink assignment control information determined from the uplink bandwidth (that is, the number of bits required for transmission) and the information size of downlink assignment control information determined from the downlink bandwidth differ. To be more specific, if an uplink bandwidth is small, the information size of uplink assignment control information becomes small, and, if a downlink bandwidth is small, the information size of downlink assignment control information becomes small. If a difference in bandwidth results in a difference in the information size like this, by adding zero information to the smaller assignment control information (that is, by performing zero-padding), the size of downlink assignment control information and the size of uplink assignment control information are made equal. By this means, whether the content is downlink assignment control information or uplink assignment control information, PDCCH signals have the same size.
The size adjustment of control information as mentioned above reduces the number of times of blind detection at a terminal on the receiving side. However, when a downlink transmission bandwidth of a base station is wide, a base station transmits many PDCCH signals at once, so that a terminal cannot reduce the number of times of blind detection much in its normal operation, and the increase of circuit scale causes a problem.
Therefore, to reduce the number of times of blind detection on a terminal more, a terminal employs the method to limit a physical region where a terminal receives control information. Thus, each terminal is reported in advance the time and frequency region that may include control information for that terminal, and performs blind detection only in a terminal-specific region where control information for that terminal is likely to be included. This terminal-specific physical region is called “dedicated region (UE SS: UE specific Search Space).” This dedicated region is associated with for example, terminal ID. Also, a time and frequency interleaving is employed to keep the effect of time diversity and frequency diversity at a certain level in the whole dedicated region.
On the other hand, a PDCCH signal includes control information that is reported at once to a plurality of terminals (for example, scheduling information about downlink broadcast signal). To transmit this control information, a physical region that is common to all terminals, called “common region (Common SS: Common Search Space),” is prepared in a PDCCH signal.
A terminal requires both control information included in a dedicated region and control information included in a common region, so that a terminal needs to perform blind detection for all of uplink control information and downlink control information included in a dedicated region and uplink control information and downlink control information included in a common region.
Also, the standardization of 3GPP LTE-advanced has been started to realize much faster communication than 3GPP LTE. 3GPP LTE-advanced system (hereinafter referred to as “LTE-A system”) adheres 3GPP LTE system (hereinafter referred to as “LTE system”). In 3GPP LTE-advanced, to realize a downlink transmission speed up to maximum 1 Gbps, a base station and a terminal which can communicate in wideband frequency of 20 MHz or more are expected to be introduced.
Also, in 3GPP LTE-Advanced, throughput requirements for an uplink and a downlink are different, so that communication bandwidths for an uplink and a downlink may be made asymmetric. Specifically, in 3GPP LTE-Advanced, it is considered to make the communication bandwidth of a downlink wider than the communication bandwidth of an uplink.
Here, a base station to support LTE-A system (hereinafter referred to as “LTE-A base station”) is formed to be able to communicate using a plurality of “component bands.” “Component band” is a bandwidth for maximum 20 MHz here and is defined as the basic unit of communication band. Furthermore, “component band” in a downlink (hereinafter referred to as “downlink component band”) is defined as a band separated by downlink frequency bandwidth information in BCH broadcasted from a base station, or a band defined by the range of distribution when a downlink control channel (PDCCH) is arranged in a distributed manner. Also, “component band” in an uplink (hereinafter referred to as “uplink component band”) is defined as a band separated by uplink frequency bandwidth information in BCH broadcasted from a base station, or the basic unit of a communication band of 20 MHz or less including a PUSCH (Physical Uplink Shared CHannel) near the center, and a PUCCH (Physical Uplink Control CHannel) for an LTE on both ends. Also, in 3GPP LTE-Advanced, “component band” may be designated as “Component Carrier (s)” in English.
FIG. 1 is a diagram showing an arrangement example of each channel in an LTE-A system where the communication bandwidth and the numbers of component bands of an uplink and a downlink are asymmetric. In FIG. 1, to let a terminal transmit an uplink signal, an LTE-A base station reports assignment control information using PDCCH from both two downlink component bands. Since an uplink component band is associated with both downlink component bands, regardless of the PDCCH whichever downlink component band is used, the PUSCH is transmitted in the same uplink band. Also, downlink assignment control information may be transmitted from both two downlink component bands, and is used to indicate downlink assignment control information in a downlink component band where each piece of downlink resource assignment information was transmitted, to a terminal.
By receiving assignment control information in this way, an LTE-A terminal can receive a plurality of component bands at the same time. However, an LTE terminal can receive only one component band at once. To group a plurality of component bands as an allocation band for single communication is called “carrier aggregation (Carrier aggregation).” This carrier aggregation can improve throughput.